A recent report about cadmium in moose livers was not surprising. Cadmium is just one of the metals that has followed a common pattern in his records and is likely to be a problem for some time to come.

Along with mercury and lead, cadmium is a natural element in Maine’s rocks and minerals and has been deposited on the landscape as dust for eons. Now it is also present in excess amounts as air pollution. Like money being put into a bank account, the pollution has deposited additional cadmium on Maine’s soils for more than 100 years, and natural processes are slowly withdrawing it. Air pollution has diminished, but it will take years of withdrawals before the account returns to the low background values of the last century.

The science story is told partly by data collected by two of my graduate students working almost 20 years apart in some of New England’s most rugged terrain. In 1979, Dennis Hanson, a master’s student with a penchant for hiking, collected soil samples along a chain of mountains from southern Vermont through Maine to Quebec.

He determined the chemical contents of the layers of soil, in particular the litter from the previous few years.

His research produced an understanding of lead, mercury and cadmium deposition in these soils across the northeastern United States and into maritime Canada. Pollution was clearly highest in southern Vermont and decreased northeasterly across Maine. The trend was similar for each metal, and the source was clearly toward the southwest. These soils were located far from factories, farms, roads and cities. The pollution had to come from the air. The metals were being deposited from the atmosphere at higher rates as a result of air pollution from human activities upwind.

That was the picture until 1996 when Chris Evans, another graduate student, decided to repeat Hanson’s work and see whether or not things had changed as a consequence of the Clean Air Act and subsequent amendments. He located the same spots, repeated the long treks and brought the samples back to Orono.

However, he faced a problem that Hanson had not. Laboratory analytical techniques had changed considerably over that 17-year span, and Evans had to figure out how to compare his results to Hanson’s. Because Hanson’s soil samples had been kept in storage, Evans was able to apply the new, more sensitive techniques to the same soils that Hanson had used. He created a straightforward formula to compare his data to what Hanson had found. He then tested the new soil samples that he had collected. Almost across the board, Evans’ results showed significant decreases in metal concentrations in the most recent layers of soil, the litter.

Clearly, pollution control is working. Lead and cadmium are almost back to natural background levels. Mercury is also down but still higher than it was before European colonists came to New England.

These results have been confirmed in other ways. Spectacle Pond in eastern Massachusetts receives most of its water from rainfall and there is little surface drainage into the lake. Thus the annually deposited sediment reflects the amount of pollutant trace metals delivered to the lake by rain and snow. Core samples taken from the pond show concentrations of the pollutant metals, cadmium included, in the sediment were low and relatively constant until about 1880 when they started to increase. The amounts of metals grew at increasing rates through the early and mid-20th century. By 1970, cadmium reached a maximum in the sediment and has since declined substantially.

In that same year, air pollution probably accounted for most of the cadmium in the organic layers of forest soil and lake sediments. The sediment in the lake told the same story as Hanson and Evans’ soil samples but with sharper focus on year to year changes.

Just like people, moose and deer receive most of their daily dose of metals from what they eat. Vegetation such as aquatic plants and mosses are the animals’ favorite foods. Little research has been done in Maine on just how much cadmium gets picked up by plants, but scientists know that Maine’s acidic soils tend to make more cadmium available to plants. Research elsewhere has shown that cadmium is taken up readily by vegetation.

The contamination of our wildlife raises concerns for the health of individual species as well as people who hunt and fish. The National Safety Council says that cadmium causes lung and testicular cancer in animals. Moreover, there is evidence that it causes prostate and kidney cancer in humans and may also cause birth defects.

Although the burden of pollutant metals is still high in soils, cadmium coming from the sky appears to be under control. However, it may be a while before we are out from under the cloud that it has created.

Steve Norton, a professor in the University of Maine’s Department of Geological Sciences, specializes in the analysis of soil and sediment samples taken from lakes, bogs and mountain slopes. This commentary was written with assistance from Nick Houtman, science writer in the UMaine Department of Public Affairs.